The invention relates to a master cylinder, often also called a master brake cylinder, for a vehicle hydraulic brake system having a housing, which has a bore, which extends along a longitudinal axis and is sealingly closed at one end by an end wall forming part of the housing of the master cylinder and at the other, opposite end by a closure element, having a piston, which is guided in the bore and extends in a sealing and axially displaceable manner through the closure element, and having a central valve, which is formed on the piston and enables a fluid connection between a fluid reservoir and a pressure chamber delimited in the bore by the end wall and the piston and which in the non-actuated state of the piston is held in open position by an abutment component acting upon the closure element, wherein the closure element is formed by two annular, axially mutually adjoining parts releasably connected to one another, of which the first part facing the bore is made of an elastomer material and the second part remote from the bore is made of a rigid material. Such a master cylinder is known from DE 196 10 834 C1, witch is incorporated by reference herein.
Master cylinders of said type are used in vehicles above all together with brake-pressure control systems, wherein nowadays said brake-pressure control systems often have the function of preventing both locking of the vehicle wheels during braking (so-called antilock system) and spinning of the driving wheels during acceleration, in particular on a slippery road surface, (so-called traction control).
In the non-actuated state of the master cylinder, i.e. when the piston is in its neutral or home position, the central valve disposed in the piston is held open by an abutment component, which is connected to the central valve and supported directly or indirectly against the end face, facing it, of the part of the closure element made of elastomer material. Upon actuation of the master cylinder, the piston with the central valve is displaced in actuating direction, wherein the abutment component detaches itself from the end face of the closure element and the central valve is closed. When after actuation of the master cylinder the piston moves back into its home position, the abutment component connected to it applies itself in a gentle and damped manner against the elastomer part of the closure element and the central valve is opened again.
The elastomer part and the part made of rigid material of the closure element are releasably connected to one another, wherein the part made of rigid material has an annular projection, which axially fully or almost fully penetrates the elastomer part. Consequently, upon a return motion of the piston into its home position the abutment component, immediately after gently applying itself against the elastomer part comes into contact with the rigid stop of the annular projection so that, irrespective of the pressure prevailing in the pressure chamber of the master cylinder, a clearly defined opening behaviour of the central valve is guaranteed.
For fastening the closure element in the bore formed in the housing of the master cylinder, at the side of the closure element facing the bore a stop plate is disposed, which is in contact with the elastomer part and is fixed in the bore by impact against a housing projection. Situated at the side remote from the bore is a snap ring, which is in contact with the part of the closure element made of rigid material and is latched in a groove formed in the inner periphery of the bore.